Amyloid P component is a 125 kDa molecular weight glycoprotein found in serum and in all types of amyloid deposits. In addition to being a normal serum protein, AP is also found in a variety of tissues. Using immunohistochemical techniques, a form of AP has been shown to be localized to normal human glomerular basement membranes, to alveolar capillary walls in sections of normal lung, and to linear intercellular structures in both cardiac and smooth muscle. Although the physiological ligand for AP in amyloid tissue is unknown, binding of AP to amyloid tissue appears to take place via the Ca.sup.2+ -dependent association of AP to heparin sulfate and/or dermatan sulfate, both minor but significant constituents of amyloid tissue. Recent studies have shown that AP is also found in association with heparin sulfate proteoglycans in cerebral amyloidosis of Alzheimer's patients. Detailed studies on the AP found in the glomerular basement membrane indicate that this form of AP is heterogeneous in nature and can only be extracted from the basement membrane by collagenase treatment. Thus, on the basis of these findings, basement membrane AP is believed to be linked to collagen or to some other integral basement membrane component(s).